Author(s): M. Yasukochi; T. Kinoshita; R. Akoh; T. Ishikawa
Linked Author(s):
Keywords: Brackish water lake; Eutrophication; Salinity Circulation; Climate change
Abstract: Lake Ogawara is a brackish water lake with a strong halocline has become eutrophicated very rapidly due to the contaminant upwelling from anaerobic hypolimnion. In this study, a numerical model of salinity circulation was constructed to explain the process of lake condition change. By referring the past field observations, three sub-processes were considered to compose the total model; (a) seawater inflow through the river under the effect of tide, (b) dilution of saltwater with lake freshwater in the process of gravity current on the lake bed slope, (c) vertical turbulent mixing and upward salinity transport in the lake. The processes- (a) and (b) were formulated being based on field observation data. The process- (c) was calculated by vertical 1-D k-εmodel taking into account the buoyancy effect caused by salinity and water temperature. Computation for ten years when the lake condition got worse explained the long-term trend of salinity condition in the lake. Major points are as follows; the depth of halocline used to be almost constant around 20meters before 2000, started to rise in 2002 and reached the depth of 14 meters in 2008. The area of halocline became three times larger, showing the increase of bottom water transport to the surface layer. As a result, surface salinity increased three times larger than the past stable condition. The simulation results also showed a good agreement with observations on seasonal changes of vertical profiles of salinity and water temperature.
Year: 2014